The present invention relates to the field of microwave radiometric (MICRAD) guidance systems. The three basic types of MICRAD guidance, shown in FIG. 1, are (1) fixing by terrain correlation, (2) target homing, utilizing tracking techniques similar to those used with radar, and (3) scene homing, utilizing target area correlation. Each guidance type has a number of advantages and disadvantages. Their pertinent characteristics are listed below. The performance figures are those that could be achieved under the most ideal conditions.
Examination of each of these three systems reveals that it would be difficult to fufill the standoff, air-to-surface missile requirements with any one system alone. The correlation fix system accuracy is limited by the quality of the reference. Routine accuracies less than 30 feet would require the highest quality references and reconnais-sance data and involve utilization of extensive complex data-processing techniques.
Correlation homing represents considerable complexity and suffers from stringent reference and reconnaissance requirements, complicated by the complex mapping geometry of the terminal phase (involving shallow-look angles and two-dimensional mapping).
Point homing has the disadvantages of limited range, and more importantly, restricted acquisition capability for targets located in complex backgrounds. If mid-course guidance results in sizable errors, there is a significant probability that the point homing system will lock onto a false target.
Although it would be difficult to meet the air-to-surface missile requirements with any one of the above techniques alone, it is possible to fulfill the requirements using both correlation fixing and point homing techniques in the same system, which system is herein described as the present invention. The correlation fix technique is utilized to get the missile within the reliable acquisition basket for point homing; and, since this may be 200 to 300 feet, the reference map requirements are greatly relaxed. If the missile can be aimed to within 200 feet of the target, the likelihood that the point homing system will lock onto a false target is very low. Therefore, the short-range limitation of the point homing system is immaterial. That is, since the course error after correlation fixing is only approximately 200 feet, the range requirement of the point homing system is small.
Various types of possible missile trajectories that can be used. Presently the pop-up trajectory is the most popular; however, each of the other kinds shown herein can be accommodated.
The present invention is a missile guidance system that utilizes a single microwave radiometric sensor for both terrain correlation and target homing. Terrain correlation is used to guide the missile to a point that is well within the acquisition and tracking range of the homing system. The homing system is then used to guide the missile to the target.
The advantages of the present invention over prior systems are that it is small, low-powered, inexpensive, passive, and capable of good target discrimination in the presence of foliage, camouflage, and sea clutter. It has superior operability that allows on-station targeting, weapon selection, 24-hour operation, and all-weather capability. And, it has target of opportunity potential and requires no access to the missile.